As well as liquid crystals having light switching function, polarizing plates having light transmitting and screening functions are basic constitutive elements of display devices such as liquid crystal displays (LCD). The fields of application of the LCD expand from small devices such as electronic calculators and watches in the early days to a wide variety of the fields such as notebook type personal computers, word processors, liquid crystal projectors, liquid crystal televisions, car navigations, and outdoor and indoor instruments, and they are used under wide conditions of low temperature to high temperature, low humidity to high humidity and low light volume to high light volume. Therefore, polarizing plates high in polarizing performance and excellent in endurance are demanded.
At present, a polarizing film is produced in the following manner. A substrate of polarizing film such as a stretched and orientated film of polyvinyl alcohol or a derivative thereof or a polyene type film obtained by dehydrochlorinating a polyvinyl chloride film or dehydrating a polyvinyl alcohol film to produce polyene, followed by orientation, is dyed or incorporated with iodine or a dichroic dye as a polarizer, thereby obtaining a polarizing film. Of these polarizing films, iodine-type polarizing films which use iodine as a polarizer are superior in initial polarizing performance, but weak against water and heat, and have a problem in endurance in case they are used for a long period of time in the state of high temperature and high humidity. In order to improve the endurance, it is proposed to treat the films with formalin or an aqueous solution containing boric acid or to use a polymeric film of low moisture permeability as a protective film, but the improvement attained by these methods is generally not sufficient. On the other hand, the dye-type polarizing films which use dichromatic dyes as polarizer are superior in moisture resistance and heat resistance to the iodine-type polarizing films, but are generally insufficient in initial polarizing performance.
In the case of a polarizing film of neutral color made by adsorbing several kinds of dichroic dyes to a polymeric film and orientating the film, if there occurs leakage of light of a specific wavelength in the visible light region (color leakage) in such a state that two pieces of polarizing films are superposed one upon another in such a manner that the orientation directions cross at right angles (crossed nicols state) and in this state, when these polarizing films are fitted to a liquid crystal panel, hue of the liquid crystal display sometimes changes in dark state. In order to inhibit change in color of liquid crystal display caused by color leakage of a specific wavelength in dark state when the polarizing films are fitted to a liquid crystal display device, it is necessary to uniformly reduce the transmittance at the crossed nicols state (transmittance at crossed nicols state) in wavelength region of the visible light region in the polarizing films of neutral color made by adsorbing several kinds of dichroic dyes to a polymeric film and orientating the film.
In the case of a color liquid crystal projection type display, namely, a color liquid crystal projector, a polarizing plate is used in the liquid crystal image forming part, and hitherto an iodine-type polarizing plate which is satisfactory in polarizing performance and shows neutral grey has been used. However, as mentioned above, the iodine-type polarizing plate suffers from the problems that it is insufficient in light resistance, heat resistance and moist heat resistance because iodine is used as a polarizer. For solving these problems, polarizing plates of neutral grey using dichroic dyestuffs as the polarizer are used, but in the case of polarizing plates of neutral grey, for averagely improving transmittance and polarizing performance in the whole visible light wavelength region, generally dyestuffs of three primary colors are used in combination. Therefore, there are problems that the light transmittance is low for satisfying the demand of market to make brighter the color liquid crystal projectors, and thus, for making brighter, the intensity of light source must be further enhanced. In order to solve the problems, three polarizing plates corresponding to three primary colors, namely, for blue color channel, green color channel and red color channel, have been used.
However, since the light is highly absorbed by the polarizing plate and images of small area of 0.9-6 inches are enlarged to about several ten inches to one hundred and several ten inches, reduction of brightness is unavoidable, and hence a light source of high luminance is used. In addition, demand for further improvement in brightness of liquid crystal projector is strong, and, as a result, the intensity of light source used is naturally further enhanced, thereby resulting in increase of light and heat applied to the polarizing plate.
As the dyes used for production of the above-mentioned dye-type polarizing films, mention may be made of water-soluble azo compounds disclosed, for example, in the following Patent Document 1 to Patent Document 8.
However, the conventional polarizing plates containing the water-soluble dyes do not sufficiently satisfy the needs of market from the viewpoints of polarization characteristics, absorption wavelength region, hue, etc. Furthermore, the three polarizing plates corresponding to three primary colors, namely, for blue color channel, green color channel and red color channel in color liquid crystal projectors are not satisfactory in all of brightness, polarizing performance, endurance under high temperature and high humidity conditions and light resistance in long-term exposure, and hence improvement in these points have been desired.
Patent Document 1: JP-A-2001-33627
Patent Document 2: JP-A-2004-137452
Patent Document 3: JP-A-11-218611
Patent Document 4: JP-A-2002-296417
Patent Document 5: JP-A-2002-275381
Patent Document 6: Patent No. 2622748
Patent Document 7: JP-A-05-295281
Patent Document 8: JP-A-60-156759